Speech communication apparatuses are known which have, in addition to a handset speaking mode for speaking with a handset, a hands-free speaking mode. In the hands-free speaking mode, speaking is done with a speaker and a microphone for hands-free speech which are provided on the communication apparatus body with a handset added thereto, that is, speaking is done with the speaker and microphone in place of the handset. In the hands-free speaking mode, the user can freely use his or her hands during speaking. In a mobile radio-telephone apparatus, for example, a driver can speak with both of his or her hands on a steering wheel. Thus safety is ensured while driving the automobile.
When, however, speaking is done in the hands-free speaking mode, the speaker's voice is reflected back from the walls and ceiling of the automobile to generate an acoustic echo around the microphone. The acoustic echo is largely not desirable on a communication system of relatively great transmission loss, in particular, due to severe degeneration of speech quality. With the digital type mobile radio telephone, for example, a low bit-rate speech encoder is employed for the effective utilization of a radio frequency. For burst errors, an interleaving system is used to enhance the burst error correction capability. For this reason, the transmission delay for speaking on one way of two-way communication apparatuses becomes about 100 msec. When communication is conducted in such a state, an echo is undesirably detected by the user, thus resulting in a large decline in speech quality.
Proposals have been made to employ an echo canceller in this kind of system. The echo canceller estimates the characteristic of an acoustic echo path through the use of an adaptive filter and generates a raise echo having the same characteristic as that on the acoustic echo path. Further, the echo canceller eliminates the false echo from a speech signal and cancels an acoustic echo component in the speech signal.
However, the echo canceller generally requires lots of time until a false echo having the same characteristic as that of the acoustic echo path is generated after the acoustic echo path has been estimated. Therefore, no adequate echo cancellation processing is carried out with the echo canceller immediately after the hands-free speaking is started. An initial echo remains immediately subsequent to the start of the hands-free speaking, thus leading to a decline in speech quality.